High-frequency continuous-time filters have traditionally utilized a basic passive integrator consisting of a transconductance stage driving a passive integrating capacitor. In the frequency domain, an ideal integrator will have a phase shift of 90.degree. because it takes 90.degree. to process the integration. According to the Laplace transform L=1/S=J .OMEGA.. Thus, s equals a 90.degree. phase shift. Unfortunately, practical circuits do not have a 90.degree. phase shift.
When an integrator is used in a filter, one simulates an inductor and a capacitor developing resonance for filtering. The quality of the filter depends upon the amount of phase shift introduced by the circuit. It is desirable for an integrator circuit to be independent of temperature, supply voltage and process variations.
FIG. 1 conceptually illustrates this structure in simplified form. An analog signal is coupled to the input of a transconductance stage G.sub.m10 which is coupled to a first terminal of an integrating capacitor C10. The other terminal of the capacitor C10 is coupled to ground. These approaches have been attractive for several reasons, among which are their simplicity, a generally smaller excess phase and power dissipation. These simpler passive integrators, however, are extremely susceptible to frequency response variations due to the parasitic capacitance associated with the parasitic-sensitive output nodes of the transconductance.
The total integrating capacitor C10 as shown in FIG. 1, comprises not only an extrinsic capacitor C.sub.1, but also of intrinsic parasitic capacitance resulting from parasitic diodes, overlaps, crossings, strays, fringing effects and so on. All of these parasitics have highly uncontrollable values, and are difficult to characterize accurately. This makes the filter much more difficult to design and manufacture. One of the consequences is that the filter requires a significantly wider trimming range in order to accurately tune the corner frequency. In addition, some parasitics, such as semiconductor junctions, are voltage dependent, which not only makes the response sensitive to power supply variations, but also degrades the distortion performance.
Parasitic capacitances associated with devices and interconnects are a particularly serious problem in high-frequency filters, where the values of integrating capacitors are small and parasitics have values that are a significant fraction of the integrating capacitor. The use of high-speed buffers at the integrator outputs can alleviate the situation somewhat, but at the expense of higher power consumption, circuit complexity, and more importantly increased excess phase shift in the resonators.
Another problem is that single-stage parasitic-sensitive passive integrators often have low open-circuit voltage gain. This results in frequency response and gain deviations that limits the practical value of Q that can be implemented.
An amplifier based active parasitic-insensitive high-gain integrator is shown in FIG. 2. Notice that the signal current from the transconductance stage G.sub.m12 is forced to flow through a capacitor C12 connected in the feedback loop of a high-gain high-speed amplifier A12. The high gain of this amplifier A12 in conjunction with its feedback loop makes the input of the amplifier behave as a virtual ground, thus preventing any signal current from flowing into, and charging the input parasitic capacitance of the amplifier circuit A12. This makes the corner frequency of this integrator insensitive to parasitic capacitance. In spite of this advantage, the use of this technique has generally been avoided in high-frequency filters because of the additional excess phase shift that the amplifier contributes. This excess phase shift is responsible for Q enhancements, and produces undesirable amplitude response and group-delay deviations. Therefore, in general, the design of practical high-frequency filters with accurate and reproducible responses, depends upon the availability of a parasitic-insensitive high-gain integrator, with negligible excess phase shift, and a well controlled unity-gain frequency.
What is needed is a continuous-time filter and an equalizer which avoids the tuning problems associated with parasitic capacitance and the excess phase shift of prior art approaches.